On Wed, Jan 31, 2018 at 11:03:39AM +0200, Oded Gabbay wrote:
On Tue, Jan 30, 2018 at 12:33 PM, Daniel Vetter daniel@ffwll.ch wrote:
On Mon, Jan 29, 2018 at 05:40:02PM +0200, Oded Gabbay wrote:
In dma_fence_release() there is a WARN_ON which could be triggered by several cases of wrong dma-fence usage. This patch adds a comment to explain two use-cases to help driver developers that use dma-fence and trigger that WARN_ON to better understand the reasons for it.
Signed-off-by: Oded Gabbay oded.gabbay@gmail.com
Not sure how useful this is, trying to do anything with a dma_fence while not holding a reference is just plain buggy. If you do that, then all kinds of use-after-free hilarity can happen. Trying to enumerate all the ways you can get refcounting wrong seems futile.
What we maybe could do is a simple one-line like
/* Failed to signal before release, could be a refcounting issue. */I think this is generally a true statement and more useful hint for the next convoluted scenario (which in all likelihood will be different from yours). -Daniel
drivers/dma-buf/dma-fence.c | 33 +++++++++++++++++++++++++++++++++ 1 file changed, 33 insertions(+)
diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 5d101c4053e0..a7170ab23ec0 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -171,6 +171,39 @@ void dma_fence_release(struct kref *kref)
trace_dma_fence_destroy(fence);
/** If the WARN_ON below is triggered it could be because the dma fence* was not signaled and therefore, the cb list is still not empty* because the cb functions were not called.** A more subtle case is where the fence got signaled by a thread that* didn't hold a ref to the fence. The following describes the scenario:** Thread A Thread B*-------------------------- --------------------------* calls dma_fence_signal() {* set signal bit** scheduled out* ---------------------------> calls dma_fence_wait_timeout() and* returns immediately** calls dma_fence_put()* |* |thread A doesn't hold ref* |to fence so ref goes to 0* |and release is called* |* -> dma_fence_release()* |* -> WARN_ON triggered** go over CB list,* call each CB and remove it* }***/ WARN_ON(!list_empty(&fence->cb_list)); if (fence->ops->release)-- 2.14.3
-- Daniel Vetter Software Engineer, Intel Corporation http://blog.ffwll.ch
Hi Daniel, Yeah, I get what you are saying. Can I add your RB to the one-liner version ?
Sure.
Reviewed-by: Daniel Vetter daniel.vetter@ffwll.ch
Cheers, Daniel